Support tabs groups

Hello,

Great extension, thank you very much for making it!

I would like to suggest just to add support for tabs groups, which can be done by simply making 1st-level subfolders in the collection. This is a very important metadata nowadays for folks who use that, and since Chrome now group tabs by default, I guess this will generalize to most users.

The issue names no files or tests. Start by locating the Svelte entry points that save tabs and sessions as bookmarks, then trace how collections map to bookmarks. Define how first-level subfolders represent tab groups and verify saving and restoring grouped tabs; no existing test location is mentioned.

Written by the indexing model from the issue text.